1. Field of the Invention
The present invention relates to a novel process for preparing 5-hydroxymethylfurfural via a 5-acyloxymethylfurfural as an intermediate. The invention also relates to a novel process for preparing the 5-acyloxymethylfurfural intermediate.
2. Discussion of the Background
5-Hydroxymethylfurfural of the following formula (I)
has, among other properties, antibacterial and corrosion-inhibiting properties and is suitable for a multitude of reactions. It is possible without any great difficulty to prepare furfuryldialcohol, -dialdehyde and -dicarboxylic acid and their derivatives therefrom; equally, the hydrogenation of the ring leads to difunctional 2,5-tetrahydrofuran derivatives. Difunctional furan derivatives substituted differently on C-2 and C-5 are also readily obtainable from 5-hydroxymethylfurfural. The aforementioned and other useful organic intermediates preparable from 5-hydroxymethylfurfural serve to prepare numerous chemical products, for instance solvents, surfactants, crop protectants and resins. In addition, the use of 5-hydroxymethylfurfural for treatment of malignant tumours has been reported (U.S. Pat. No. 5,006,551).
5-Hydroxymethylfurfural is an intramolecular, triple dehydration product of hexoses (aldohexoses and ketohexoses). Renewable raw materials, such as starch, cellulose, sucrose or inulin, are inexpensive starting substances for preparing hexoses, such as glucose and fructose. 5-Hydroxymethylfurfural is in principle an intermediate of the dehydrating decomposition of hexoses to laevulinic acid and formic acid, i.e. what is crucial is to stop the reaction at the correct time. This makes the removal of 5-hydroxymethylfurfural from the starting sugars and by-products an important step in its preparation.
A large number of different processes for preparing 5-hydroxymethylfurfural on the laboratory scale is already known.
The catalysts which have been described for the dehydration are different acids or salts, for example oxalic acid (cf. W. N. Haworth et al., J. Chem. Soc. 1944, 667), salts such as pyridine hydrochloride (cf. C. Fayet et al., Carbohydr. Res. 122, 59 (1983)), acidic ion exchangers (cf. DE-A-30 33 527) or Lewis acids such as zirconyl chloride (cf. SU-A-1 054 349, cit. CA 100, 120866s) or boron trifluoride etherate (cf. H. H. Szmant et al., J. Chem. Tech. Biotechnol 31, 135 (1981)).
For the industrial scale preparation of 5-hydroxymethylfurfural the catalyst used should be inexpensive and non-corrosive. Solid catalysts intended for reuse are, owing to the easy manner of formation of insoluble by-products, unsuitable because a removal of the catalyst (e.g. ion exchanger) from these by-products is uneconomic or impossible.
Lewis acids such as zirconyl chloride or aluminium chloride likewise have to be rejected on the basis of considerations relating to corrosion protection. The use of sulphuric acid or phosphoric acid is therefore considered to be favourable, since the acidic aqueous reaction solutions can optionally be neutralized with bases in this case, and, for instance in the case of use of calcium hydroxide or calcium carbonate, the conversion of the catalyst acids to sparingly soluble salts with removal by filtration is possible.
The reaction medium of the dehydration of saccharides is determined by their solubility. In addition to water, dipolar aprotic solvents in particular, such as dimethylformamide or dimethyl sulphoxide, have been used.
The iodine-catalysed conversion of the fructose portion of sucrose to 5-hydroxymethylfurfural by heating sucrose in anhydrous dimethylformamide entails, as well as the expensive solvent, also a complicated workup, specifically extraction and paper chromatography (cf. T. G. Bonner et al., J. Chem. Soc. 1960, 787).
In the decomposition of fructose with different catalysts, good yields (>90%) of 5-hydroxymethylfurfural are found in dimethyl sulphoxide (cf. H. H. Szmant et al., J. Chem. Tech Biotechnol. 31, 135 (1981)). However, the isolation of the desired product is difficult owing to the high boiling point of the solvent among other reasons, and entails a multistage extraction.
DE-A-33 09 564 therefore proposes, for the isolation of the 5-hydroxymethylfurfural from solutions comprising dimethyl sulphoxide, a derivatization to 5-acetoxymethylfurfural. As well as a vacuum distillation, this also entails two reaction steps (acetylization, deacetylization) and hence consumption of time and chemicals.
Several processes use mixtures of water and organic solvents as a reaction medium. In U.S. Pat. No. 2,929,823, furfural is added to aqueous saccharide solutions and heated briefly (0.1-120 s) to 250-380° C. Tar-like by-products are dissolved by the organic solvent added, as is 5-hydroxymethylfurfural. The preparation of 5-hydroxymethylfurfural in pure form thus appears to be performable only with difficulty.
A further biphasic process is described in DE-A-30 33 527. In this process, under relatively mild conditions (below 100° C.), fructose-containing aqueous solutions are decomposed with acidic cation exchangers, an organic solvent which is not water-miscible but nevertheless has a good distillation capacity for 5-hydroxymethylfurfural being present. The great disadvantage of this process is that a very large excess of the organic solvent, based on the aqueous phase (>7:1), is needed, and the solvents required are expensive and toxic. Moreover, the very good solubility of 5-hydroxymethylfurfural in water makes any extraction of the product with organic solvents from aqueous solutions exceptionally difficult.
The publication by C. Fayet et al., Carbohydr. Res. 122 (1983), 59, describes the decomposition of saccharides without solvent, but with equimolar amounts of catalyst. The pyridine hydrochloride catalyst is, however, unsuitable for an industrial use of the process. Furthermore, addition of water is followed by a laborious extraction (20 h) with ethyl acetate.
This is also expressed in the publication by D. W. Brown et al., J. Chem. Tech. Biotechnol. 32, 920 (1982), where it is stated analogously that the more recent methods of preparing 5-hydroxymethylfurfural have the disadvantage that the product is present in the aqueous phase or in a polar solvent, from which the isolation is difficult.
EP-A 0 230 250 describes a process for preparing 5-hydroxymethylfurfural, in which saccharides are decomposed with an acidic catalyst in aqueous solution, optionally with addition of organic solvents, above 100° C. After removing any solvents present, the reaction mixture is chromatographed by means of ion exchange columns with exclusive use of water as the solvent, a mixed fraction obtainable when a relatively large amount is applied is rechromatographed, and the 5-hydroxymethylfurfural is crystallized out of the corresponding fractions. Although the process described purportedly leads to 5-hydroxymethylfurfural in high purity, the multiple chromatography and crystallization is too complicated and expensive on the industrial scale.
Further processes for preparing 5-hydroxymethylfurfural are disclosed, for example, in Bull. Soc. Chem. France 1987, 5, 855; J. Chem. Tech. Biotechnol. 1992, 55, 139; FR-A-2 669 635; FR-A-2 664 273; U.S. Pat. Nos. 3,483,228; 2,917,520; 3,071,599; 3,066,150 and 2,750,394.